The present invention relates generally to thyristors used in electronic switching circuits and power controllers and relates more particularly to silicon controlled rectifiers and triacs used as a controlled switch in commercial and household electronic devices for applying power from AC power lines.
Thyristors are integrated-circuit components that are used to control the application of electrical power to a load. In effect, they function as electronic switches. Triacs and silicon controlled rectifiers (SCRs) are varieties of thyristors. The triac functions as a bidirectional alternating-current switch, and the SCR functions as a uni-directional switch permitting rectified current flow in one direction only. Triacs and SCRs are found in many industrial and consumer products in such uses as light dimmers, motor speed controllers, heater controllers, or simply controlled switches for applying electrical power in response to the happening of some event.
The problems addressed by the present invention will be more readily understood if discussed in a specific example. They are illustrated here in the example of a passive infrared (PIR) motion detector, although the same problems and concerns arise in a variety of applications. A PIR motion detector is a device used in automatic light switches and security systems to turn on a light or to activate some other form of alarm or warning indicator when a person or motor vehicle enters a monitored area. Such devices are used with outdoor residential lighting, for example, to illuminate a walkway as a person approaches the front door or to illuminate a driveway as a car approaches.
The PIR motion detector functions by sensing heat in the form of infrared radiation emanating from a person or similarly warm object as the person or object enters or moves about in the field of view of the detector. When the motion detector detects an appropriate heat impulse characteristic of a moving heat source, it provides an electrical signal to activate the light or other alarm. This electrical signal is generally a low-voltage signal, typically on the order of 5 to 15 Volts, that does not carry sufficient power to energize the typical electric lights used for interior or exterior building lighting. Household lighting is generally designed to be energized at 120 Volts. This is the nominal voltage of the electrical service provided by local utility companies throughout the United States. To overcome this deficiency, the low-voltage signal developed by the motion detector (sometimes referred to as a triggering or gating signal) is applied to a switching component, which responds by permitting electric current to flow through the lights at the higher 120-Volt level of the power line. The triac is a common switching component used for this purpose, and the SCR is often used where uni-directional current is desired.
While the triac and the SCR have proven to be reliable and popular switching components in motion detectors and other electronic products, they nevertheless have their limitations. Usually a special firing circuit is needed to "fire" or gate the triac (SCR) so that it conducts at the higher voltage of the power mains. The firing circuit desirably provides a "clean" gating signal that is comparatively insensitive to line fluctuations, noise transients or other irregularities to provide unambiguous triggering of the triac (SCR). In addition, the triac and SCR do not necessarily pass all the power of the line current through to the load. The average power that the triac (SCR) applies to the load depends on when the triac (SCR) is fired during the alternating current cycle. Triac and SCR firing circuits commonly include a phase-control circuit and may also include a zero-crossing detector for establishing a desired phase relationship between the triac (SCR) gate signal and the applied AC line voltage. This phase relationship determines the proportion of the AC waveform (and hence the average power) that is applied to the load per AC cycle.
The need for a triac (SCR) firing circuit ultimately increases cost of the product. More complicated firing circuits generally require a greater number of circuit components and make greater demands on product assembly or are provided by more costly specialized integrated-circuit chips. For low-cost household and consumer products the extra cost of the triac (SCR) firing circuit is significant and increases the price at which the product may be offered to the end user. Nevertheless, in the past the need for special (and often complex) thyristor firing circuits for efficient and reliable thyristor operation could not be avoided.
Another constraint on the use of thyristors is the need for precautions against electrical shock. For a household or commercial product to receive approval from regulatory safety-testing organizations, the 120-Volt line voltage must be properly isolated within the product to prevent injury in the event of a malfunction. Thus, for example, to achieve efficient triac firing while sufficiently isolating the triac firing circuit and the rest of the product's low-voltage circuitry from the 120-Volt line voltage, specialized integrated-circuit triac firing chips have been developed, such as the MOC 3010 optical driver. This chip fires the triac reliably at the desired time (i.e., phase relationship) and provides an optical link between the low and high-voltage circuits that protects quite well against dangerous, undesired electrical connections between the two circuits. The MOC 3010 optical driver chip successfully meets the dual technical demands of reliable, precise triac firing and safety. For use in a low-cost household or commercial product, however, it has the disadvantage of high cost.